test git version string

The git version string is used in the wild, and in git gui,
for capability checking purposes. Test the string format
and version X.Y.Z number.

Signed-off-by: Philip Oakley <philipoakley@iee.org>
Doc: State the exact git version string

Provide the regular expression that matches the
'git --version' strings.

Scripts in the wild, including 'git gui', check the git version.

Signed-off-by: Philip Oakley <philipoakley@iee.org>